A gallium nitride (GaN) compound is a material that emits a light having a wavelength of various regions ranging from a visible light to an ultraviolet ray. The gallium nitride compound is widely used for an electrical device such as a large output/high temperature operation FET (Field Effect Transistor) and a HEMT (High Electron Mobility Transistor) as well as an optical device. Recently, the gallium nitride compound is used in manufacturing a blue LED (light emitting diode).
FIG. 1 is a perspective view illustrating a structure of a gallium nitride-based LED of a conventional art.
Referring to FIG. 1, the LED comprises a sapphire substrate 11, an n-type gallium nitride film 13, an active layer 15, a p-type gallium nitride film 17, a transparent electrode 19, an n-electrode 21 and a p-electrode 23.
A manufacturing process of the LED usually utilizes a MOCVD (Metal Organic Chemical Vapor Deposition) apparatus to grow the gallium nitride film on the sapphire substrate 11. Firstly, a buffer layer for aiding the growth of the gallium nitride film is formed on the sapphire substrate 11. Thereafter, the n-type gallium nitride film 13, the active layer 15 and the p-type gallium nitride film 17 are sequentially formed.
When an electric current flows through the n-electrode 21 and the p-electrode 23, a recombination of a hole and an electron occurs in the active layer 15 to emit a light. While an electrode is disposed on top of a p-layer and another electrode is disposed on a back of a substrate coupled to an n-layer in a conventional diode such that the electric current flows to a p-n junction, the electrode cannot be disposed on a back of the sapphire substrate 11 in the gallium nitride-based diode, which is an insulator. Therefore, the electrode has to be disposed directly on the n-type gallium nitride film 13.
Accordingly, the n-electrode 21 is disposed on the n-type gallium nitride film 13 which is exposed by etching a portion of the p-type gallium nitride film 17. Since the light is emitted from the p-n junction, the p-electrode 23 is disposed at an edge of the p-type gallium nitride film 17 such that the light is not blocked by the electrode.
While the gallium nitride film is disposed on the sapphire substrate 11, the film disposed on the sapphire substrate 11 is not restricted to the gallium nitride film. For instance, the gallium nitride film can be grown on a silicon carbide substrate.
A most important factor determining an electrical characteristic of a nitride light emitting diode is a characteristic of an ohmic contact between the p-type gallium nitride film and a metal layer. When a characteristic of the ohmic contact is poor, an operating voltage of the device is very high, thereby degrading a reliability of the device.
The ohmic contact refers to an area where two materials are in contact having a characteristic wherein the electric current flowing in a contacting portion is proportional to an electric potential difference of the contacting portion.
Methods for improving the characteristic of the ohmic contact of the GaN semiconductor device have been proposed. In accordance with one method, a Pt metal material having a high work function is used to lower a schottky barrier between a p-type gallium nitride interface. In accordance with another method, a compound reactive layer is formed by subjecting an interface between the p-type gallium nitride and a metal to a thermal process. For instance, a compound such as a NiO or a NiGax is formed at the interface using Ni/Au layer, or a PdGax compound is formed using a Pd electrode. A gallide compound such as the PdGax generates a vacancy of the gallium in the p-type gallium nitride, which serves as the hole.
Therefore, a tunnel junction effect wherein a height of a barrier is reduced by forming a high concentration of the hole at a surface of the p-type gallium nitride contacting the metal is obtained, thereby improving the characteristic of the ohmic contact. When a device is actually implemented, the two effects is obtained in a combined fashion.
However, the methods are limited in obtaining a sufficient characteristic of the ohmic contact because a vacancy of the nitrogen is also generated at the surface of the p-type gallium nitride during the heat treatment as well as the vacancy of the gallium. The vacancy of the nitrogen reduces a hole concentration contrary to that of the gallium.
Therefore, in order to obtain an optimum characteristic of ohmic contact, a method for increasing the hole concentration without generating the vacancy of the nitrogen in the p-type gallium nitride during the heat treatment is necessary.